Many industrial applications require precise determination of the volume of fluid flowing through a measuring line. The volume of fluid can be calculated from the cross section of the measuring line and the flow speed of the fluid. Determining the cross section of the line is easy, since it is preset. Hence, precise measurement of flow speed is decisive in accurately determining flow volume. On the one hand, the measured value of the flow speed must be precise and, on the other hand, it may be necessary to take a continuous measurement or repeat measurements at the shortest possible time intervals, since short-term fluctuations in flow speed can occur. In order to include these fluctuations in the calculation of the flow volume over a longer period of time, the total flow volume is determined by adding the flow volume within short periods of time.
In the state of the art (H. Bernard, "Ultraschall-Durchflussmessung" in Das Handbuch fur Ingenieure", Bonfig/Bartz/Wolff, 2nd Edition), on which the invention is based, the flow speed of the fluid is measured by sending an acoustic signal along a set measuring path. In this way, the timing of the acoustic signal on the measuring path from an acoustic sender to an acoustic receiver in the fluid is determined by the sound velocity and the flow speed (drag). The principle of acoustic flow measurement by the time difference method is derived from this. In the fluid, acoustic signals are sent upstream and downstream alternately or simultaneously. Because of the different diffusion speed, the acoustic signals reach the acoustic receiver after different times t.sub.1 and t.sub.2 upstream and downstream on the same length measuring path. The difference t.sub.2 -t.sub.1 is a measurement of the average flow speed on the measuring path formed by the acoustic sender and receiver. The senders are excited to oscillate by an electrical voltage and give off an acoustic signal in the fluid. The acoustic receivers receive this signal after it runs along the measuring path and convert it into an electrical voltage. The time difference is detected from the acoustic signal times found in this way, and is proportional to the flow speed of the fluid in the measuring line.
In the state of the art, two measuring heads are placed on a measuring line in such a way that their connecting line has a component parallel to the direction of the flow speed. The measuring heads are either placed in contact with the fluid in the measuring line or attached to the measuring line from outside with no contact with the fluid. The measuring heads each contain a piezoelectric transducer, with which the acoustic signal necessary for flow measurement is produced and received (acoustic receiver). If a high-frequency alternating voltage is applied to the two surfaces of the piezoelectric transducer that have electrically conductive layers, the piezoelectric transducer periodically changes its thickness at the same frequency and is capable of producing sound waves in the surrounding medium. This process is reversible, so that acoustic signals running from the sender through the fluid to a piezoelectric transducer of the second measuring head produce a thickness oscillation therein which produces an electrical alternating voltage in the piezoelectric transducer of the second measuring head. This is amplified and further processed by known electronic elements.
Moreover, in the state of the art, direct time measurement by the "leading edge" method is used for the time difference method. For this, a precisely defined, pulsed acoustic signal is sent from a first measuring head to a second measuring head, wherein to measure the time t.sub.1, only the first sharp, precisely definable side of the pulsed acoustic signal is used. At the same time, an acoustic signal is sent from the second measuring head to the first measuring head and the time t.sub.2 is measured in the same way. The time difference t.sub.2 -t.sub.1 is directly linearly proportional to the average flow speed; other parameters like, for example, the temperature-dependent density and viscosity, are not included in the measurement.
In addition to the acoustic signal sent from the fluid as a measuring signal, an acoustic signal occurs as an interfering signal due to the transmission of the acoustic signal through the material of the measuring line. In the known volume flow meters, the measuring line is made of metal, in which the sound velocity is greater than in fluid. The sound velocity for metals is in the range of 4,000-5,000 m/s and for fluids in the range of 1,500 m/s. The acoustic signal to be regarded as an interfering signal is thus received by the acoustic receiver before the acoustic signal to be evaluated as a measuring signal, so that the measuring signal is superimposed over the interfering signal. This superposition thus occurs especially at the beginning of the measuring signal used as a measurement for determining the running time, as described above. The intensity of the interfering signal is generally the same size or greater than the measuring signal, since the acoustic signal is transmitted very well through metal. This also makes it difficult to determine the running time of the measuring signal.
In the state of the art, attempts have been made in various ways to suppress the interfering signal using evaluation technology. One possibility consists of the fact that a time window is set up in which the interfering signal occurs regardless of the flow speed at constant running time, while the acoustic signal received by the respective acoustic receiver is suppressed. But it must be guaranteed that, in each case, there is a sufficient running time difference between the interfering signal and the measuring signal. In the state of the art, the known volume flow meters must, therefore, have a long enough measuring path. Another way of suppressing the interfering signal is by setting an intensity threshold from which the output signal produced by the acoustic receiver is evaluated. Here, however, it must be guaranteed that the intensity of the measuring signal is greater than that of the interfering signal. But this is problematic, as already described above.